It is known that a plasma treatment of surfaces increases the adhesiveness of various materials during a subsequent coating, like for example a subsequent lacquering.
German patent publication DE 199 57 775 C1 discloses a device for plasma treatment of wood, wherein the wood is earthed to serve as a counter-electrode for the electrode having the dielectric barrier, to which the AC high voltage is applied.
From international patent application publication WO 2004/105810 A1 is it known to treat biologic materials containing living cells with a plasma. Here, the gas discharge is ignited between the electrode with the dielectric barrier and the biologic material by means of an AC high voltage applied to the electrode, wherein the biologic material serves as a capacitive counter-electrode for the electrode with the dielectric barrier. The electrode comprising the dielectric barrier which covers the electrode has a tapering shape. The device known from WO 20041105810 A1 is provided as a battery- or accumulator-powered, hand-held unit, its AC high voltage source being based on semiconductor technology.
If a plasma for a surface treatment is generated by means of a dielectric barrier discharge at atmospheric pressure, this is up to now always done between an electrode with a dielectric barrier and some kind of a counter-electrode which is located at a certain distance to the electrode with the dielectric barrier, the gas in which the discharge is ignited being present between the electrode and the counter-electrode.
International patent application publication WO 87/07248 A1 discloses a device for treating objects using electric high voltage discharges in a gaseous media. Here, a unipolar AC high voltage is applied to an electrode which comprises a number of needle-shaped extensions extending in parallel to each other and embedded into a dielectric. Open channels run through the dielectric in parallel to the needle-shaped extensions, through which a spark discharge, i.e. no dielectric barrier discharge, extends up to the main part of the electrode. The electrode is arranged in front of the objects to be treated which are contacted electrically to provide a counter-electrode for the electrode.
International patent application publication WO 02/065820 A1 discloses a device for plasma treatment at atmospheric pressure, which has two opposing electrodes. One of the electrodes is earthed and provided with a dielectric barrier covering the electrode completely. A dielectric in front the other electrode which is connected to an AC high voltage source to apply an AC high voltage comprises open discharging gaps into which conductor electrodes extend from the other electrodes. The conductor electrodes are provides with pointed tips pointing towards the earthed electrode. Due to the concentration of the field strength of the electric field between the two electrodes at the pointed tips of the conductor electrodes, a gas discharge is ignited in a gas present between the two electrodes. This gas discharge is only dielectrically barred by the dielectric in front of the earthed electrode, as the dielectric in front of the electrode connected to the AC high voltage source comprises the open discharge gaps. The known device in constructed to introduce only gas in the space between the two electrodes. The plasma generated in the space between the electrodes may be used to treat surfaces located outside this space.
German patent application publication DE 197 17 698 A1 discloses a device for cleaning or activating electric circuit paths and surfaces of printed circuit boards. This device comprises a pair of opposing electrodes, at least one of which is provided with a dielectric barrier covering it completely. Emission tips which enhance the ignition of a gas discharge between the electrodes and homogenize the gas discharge are formed at the outer surface of one of the electrodes or of its dielectric barrier. The objects to be treated are introduced between the electrodes of the known device and may rest on one of the electrodes or its dielectric barrier. Preferably only the opposing electrode has emission tips. The emission tips may for example be made by etching the respective surface of a dielectric material. This results in radiuses of curvature of the tips of about 1 μm. Generally, the radiuses of curvature of the emission tips are between 10 nm and 0.5 mm. The needle- or pin-shaped emission tips may be provided in a surface density of between 1 and 100 per cm2. Upon movement of the object to be treated into the space between the electrodes, there is a danger of contact of the object with the sharp emission tips extending into this space. Due to this contact the objects to be treated and/or the emission tips may be damaged.
The devices described up to here are only poorly or even not at all suited for treating materials which are poor or even no electric conductors, like for example plastics, glasses and stones or even dry wood, as these materials may not effectively serve as counter-electrodes for the electrode with the dielectric barrier.
Devices for generating potential-free plasmas in the form of a plasma beam, which are sometimes designated as plasma-jets or plasma-blasters, are commercially offered. Such devices base on a different principle than a dielectric barrier discharge. As rule, they need a power grid connection or, at least, a gas connection. Further, these devices are very expensive.
There is a need of a device for plasma treatment at atmospheric pressure which can be provided at low cost which has no sharp tips at an outer surface, and which nevertheless enables a plasma treatment of poorly or even not at all electrically conducting materials.